1. Field of the Invention
The present invention relates to the electrical treatment of biological tissue. In particular, the present invention discloses a device that produces discrete electrical pulse trains for treating osteoporosis and accelerating bone growth.
2. Discussion of Background
Human bone is a combination of organic and mineral components; the chief mineral present is hydroxyapatite, a complex calcium phosphate (Ca.sub.5 (PO.sub.4).sub.3 OH) in crystalline form. Hydroxyapatite is piezoelectric; that is, it generates an electric charge when mechanically stressed. The electric signals generated by hydroxyapatite under stress, sometimes referred to as "bone talk", are detected by nearby bone cells, stimulating them to increase the production of hydroxyapatite. This increase in production of hydroxyapatite due to "bone talk" appears to be part of a feedback mechanism causing bone to be strengthened automatically at points of stress concentration. This feedback mechanism is weakened or interrupted in areas surrounding a bone fracture since stress concentrations at the fracture are typically nil.
A similar feedback mechanism seems to control the mineral content of intact bone. When normal "bone talk" is no longer communicated to surrounding cells, the production of hydroxyapatite decreases and osteoporosis (meaning "brittle bones") can result. Restoration of the piezoelectric signal can slow or reverse this condition.
It has long been known that the application of electric currents could speed bone growth and repair, but it was not until the 1960's that the weak signals generated by the bone itself were measured, analyzed, stimulated, and used to aid in healing. Studies conducted by C. A. L. Basset and others resulted in the identification of optimum waveforms which essentially duplicate the electrical characteristics of normal "bone talk".
FIG. 1 shows the optimum waveform which replicates normal "bone talk" instrumental in healing fractured bones. A series of pulses 10 consists of pulses 12 with pulse width 14 (5 msec), amplitude 16, and pulse interval 18 (61 msec) for a frequency of about 15 Hz. Each pulse 12 contains subpulses 20 with subpulse width 22 (200 .mu.sec) and subpulse interval 24 (28 .mu.sec) for a frequency of about 4400 Hz. A waveform used for treatment of osteoporosis, shown in FIG. 2, consists of a series of pulses 30, with pulses 32 of pulse width 34 (380 .mu.sec), amplitude 36, and pulse interval 38 (13.5 msec) for a frequency of about 72 Hz.
While it was initially thought that synthetic "bone talk" signals would have to be relatively strong if applied from outside the body, it now seems that a threshold effect is involved. It has been found that alternating current signals billions of times more powerful than Bassett-type signals have virtually no greater effect on the healing rate of fractured bones than do their weaker counterparts. In addition, non-Bassett type alternating current signals may be used, but such signals generally require much higher power levels for equal effectiveness. Furthermore, the use of direct current signals is itself undesirable because they can result in electrolytic tissue damage and, if the signal strength is too great, actually destroy bone structure.
Optimum waveforms, generated by alternating current signals, can nearly double the rate of bone healing in ordinary fractures, and restart healing in nonunion fractures, i.e. those fractures in which normal healing has stopped without rejoining the pieces of the broken bone. The conventional treatment of non-union fractures involves surgical procedures which are often unsuccessful and invariably increases both the discomfort and expense incurred by the patient. The use of electronic stimulation as a method of treating fractured bones has reduced the reliance on conventional surgery.
Medical uses of electrical stimulation have been limited, possibly due to a belief that only high powered external signals will be effective. Direct transmission of high powered signals through the skin is harmful, potentially causing burn-like electrolytic damage. One alternative, electrode implantation below the skin, requires surgery. Such invasive procedures bring additional stress to the patient and, because of the possibility of infection, require ongoing medical attention. Also, such electrodes will require continuing adjustment since the current will be concentrated near the electrodes rather than being evenly distributed throughout the afflicted area.
As a result of limitations in present design, most modern work in electrical bone growth stimulation has been through the use of induction coils. See Niemi (U.S. Pat. No. 5,548,208). Radio-frequency (R.F.), signals applied to these coils, which are placed against the skin or on a plaster cast, induce signals of similar form in bone and other tissue. This method is non-invasive, simplifies patient care, and has the advantage of transmitting A.C. signals while D.C. signals are blocked. However, induction coil type stimulation is very ineffective at low "bone talk" frequencies. As a result, elaborate circuit modifications such as deliberate distortion or frequency modulation are required to approximate natural "bone talk" frequencies. Alternatively, circuit simplicity can be retained at the cost of an added power drain by using non-Bassett type signals such as sine waves.
Coils, high-frequency generators, modulating and driving circuitry all add to the weight, bulk, cost and power requirements of the signal generator. Generators now in use typically cost several thousand dollars each. Nominally "portable" equipment is driven by heavy, rechargeable battery packs with limited capacity while stationary devices require connection to A.C. power. The limitations of using existing stimulators require intermittent use by patients, typically for three to eight hours a day. Some models include circuitry to monitor use and insure patient compliance, but such circuitry increases the cost of the equipment. To ensure proper field distribution and adequate signal penetration into the bone, induction coils must frequently be custom made to fit the patient, thereby requiring adjustments to the signal generator. These modifications also add to cost and often delay the beginning of treatment.
Therefore, there is a present need for an electrical bone growth stimulator that generates Bassett-type signals, comparable to the pulse trains shown in FIG. 1 and FIG. 2. Such stimulator should be lightweight, compact, fully self-contained, inexpensive to manufacture and maintain, safe for unsupervised home use, and require no external coils or battery packs.